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A 75-million-year old fossil shows a group of ancient birds could fly as well as their more successful peers, so why did they die out? Samantha Page reports.

Scientists have completed an analysis of a rare, nearly complete fossil from a group of birds that went extinct, finding they were “aerodynamic equals” to the ancestors of today’s birds.

The 75-million-year old fossil, about the size of a turkey vulture, raises new questions about why the group, known as enantiornithines, died out while euornithes, ancestors to all modern bird species, survived, the researchers report in the journal PeerJ.

“What this new fossil shows is that enantiornithines, though totally separate from modern birds, evolved some of the same adaptations for highly refined, advanced flight styles,” says Jesse Atterholt, a former doctoral candidate at University of California, Berkeley, where the fossil is housed.

Another study, published earlier this year, described the 127-million-year-old fossil of an enantiornithean chick that had died before its skeleton turned from cartilage to bone. It would thus not have been able to fly and may have been dependent on its parents for food and shelter – a situation similar to that of many modern hatchlings.

The term “enantiornithines” means “opposite birds”. The group died out around 65 million years ago at the end of the Cretaceous period, but scientists are not sure why.

"One of the really interesting and mysterious things about enantiornithines is that we find them throughout the Cretaceous, for roughly 100 million years of existence, and they were very successful,” Atterholt says.

“We find their fossils on every continent, all over the world, and their fossils are very, very common, in a lot of areas more common than the group that led to modern birds. And yet modern birds survived the extinction while enantiornithines go extinct."

Atterholt and Berkeley-based palaeontologist Howard Hutchison, who found the fossil more than 25 years ago, worked with Jingmai O'Conner, an enantiornithines expert, to conduct the new analysis. She and her colleagues named the new species Mirarce eatoni.

At first glance, the animal would have looked very much like a modern bird, she says. It was fully feathered and flew by flapping its wings. Unlike today’s birds, though, the wings likely sported claws -- “ideal for perching and perhaps climbing,” says Atterholt. The beak contained teeth.

The inferred perching and climbing behaviours suggest that enantiornithines were forest dwellers. This has led to the idea that when the forests died out at the end of the Cretaceous, enantiornithines went with them.

“I think it is a really interesting hypothesis and the best explanation I have heard so far,” Atterholt says. “But we need to do really rigorous studies of enantiornithines' ecology, because right now that part of the puzzle is a little hand-wavey.”